An optical head device provided with an optical element such as a retarder, a diffraction element and so on has been used in order to write (Hereinbelow, referred to as record) information in an optical recording medium such as an optical disk, e.g., CD, DVD or the like, or a magneto-optical disk (Hereinbelow, referred to as an optical disk), or read out (Hereinbelow, referred to as reproduce) information from the optical recording medium.
The optical head device is to introduce laser light emitted from a semiconductor laser light source to the optical disk by converging the laser light by means of an objective lens, and to detect information recorded in the optical disk by receiving reflected light from the optical disk by a photodetector.
In an optical path from the semiconductor laser source to the optical disk or an optical path from the optical disk to the photodetector, an optical element such as a diffraction element, a beam splitter or the like and a retarder for changing a state of polarization of laser light are properly arranged to obtain highly accurate, stable recording/reproducing. In particular, by using a combination of the optical element in which the characteristics are changed depending on a state of polarization of laser light and the retarder for changing a state of polarization, an optical system having a high utilization efficiency of light can be realized, and flexibility in designing the optical system can be improved. Accordingly, various types of optical system for optical head devices have been proposed.
For the retarder, such one prepared by polishing a single crystal of inorganic substance such as quartz has conventionally been used. However, since the single crystal of inorganic substance has large incident angle dependence on a phase difference of transmitted light, the single crystal is improper as the retarder for generating a desired phase difference. Further, in an optical element made of the single crystal of inorganic substance, the number of steps in manufacture is large. Accordingly, such one having a thin film of organic material made of, for example, uniaxially stretched polycarbonate, having birefringent properties has been proposed. In cases of using the conventional retarder formed by polishing the single crystal of inorganic substance such as quartz and the retarder comprising an organic material, there were the problems as described below, in common.
As a first problem, there is the temperature dependence of the wavefront aberration of transmitted light from the retarder. When an optical head device is operated for a long time, the temperature in the optical head device rises with a lapse of time. Further, a semiconductor laser source used for the optical head device has generally such temperature characteristics that the oscillated wavelength increases with a temperature rise. In the optical head device installing therein the retarder, an oscillated wavelength from a semiconductor laser source fluctuates due to a temperature change, a predetermined phase difference at the time when the laser light passes through the retarder can not be obtained, and an adverse influence such as a reduction of signal intensity or an increase of noises in signals may be caused in recording/reproducing an optical disk.
In the quartz or the thin film of organic material, the fluctuation of the optical characteristics due to a temperature change is generally small. Accordingly, when the oscillated wavelength from the semiconductor laser source fluctuates due to a temperature change, a fluctuation in the wavelength of the laser light could not be compensated by the retarder in which the above-mentioned material was used.
Further, since the linear expansion coefficient of a fixing substrate used in the retarder was generally different from the linear expansion coefficient of a thin film of organic material, the thin film of organic material deformed with a temperature rise, disturbances in the retardation value of the retarder and the retardation axis (fast axis) direction took place, and further, the smoothness of the thin film of organic material was reduced, whereby there was a problem of causing the disturbance of the wavefront of the laser light passing through the retarder.
As a second problem, it was difficult to minimize the size of the device because of an increased number of parts in assembling the retarder in the optical head device. In the retarder formed by polishing the single crystal of inorganic substance such as quartz, the retarder had to be located in a region of parallel light in an optical path in the optical head device because the incident angle dependence of phase difference was large, whereby there was a problem that the surface area of the retarder became large. Further, in the retarder made of an organic material, although the incident angle dependence of phase difference was small, the thickness of the retarder formed by bonding on the substrate was increased as a whole. Accordingly, it was difficult to minimize the size and the weight of the optical head device.
As a third problem, there is the wavelength dependence in a state of polarization of transmitted light in the retarder. Since the recording density can be increased by shortening the wavelength of usable laser light in an optical head device, an attempt of shortening the wavelength has been made on light sources. On the other hand, it is necessary to perform reproduction by using widespread laser light having a wavelength band of 790 nm for many compact optical disks (Hereinbelow, referred to as CD), and various optical head systems which allow the compatibility between laser light having a shorter wavelength and the ordinary laser light having a wavelength band of 790 nm have been proposed. In order to assure the compatibility with respect to the ordinary optical disks, there is a system in which the ordinary laser light source having a wavelength band of 790 nm is disposed in addition to a short wavelength laser light source having a wavelength band of 650 nm for digital versatile disks for high-density recording (Hereinbelow, referred to as DVD).
Further, as a semiconductor laser source for emitting light having two-wavelengths, a two-wavelength semiconductor laser source of monolithic structure in which a semiconductor laser source having a wavelength band of 790 nm and a semiconductor laser source having a wavelength band of 650 nm are formed in one chip, or a two-wavelength semiconductor laser source comprising a plurality of chips in which laser chips having different wavelength bands are disposed with an interval of light emitting point of about 100-300 μm, for example. With such two-wavelength semiconductor laser sources, the number of parts can be reduced to minimize the size in comparison with conventional optical head devices having two-semiconductor laser sources as separate units. Further, in the case of the two-wavelength semiconductor laser source of monolithic structure, accuracy in positioning light emitting points can be improved, and assembling and adjusting are simplified, whereby the characteristics of optical head capable of providing stable recording/reproducing information are easily obtainable.
In a case that a retarder is used for a conventional optical head device using only a single wavelength, it is easy to prepare a quarter-wave plate providing a phase difference of π/2 or a half-wave plate for providing a phase difference of π with respect to linearly polarized incident light. However, when a retarder is used for an optical head device using two-wavelengths, it was difficult for the conventional retarder to provide optical characteristics of such technical level that the quarter-wave plate was usable for a wavelength band of 650 nm and a wavelength band of 790 nm, or the half-wave plate was usable for a wavelength band of 650 nm and a wavelength band of 790 nm.
Accordingly, for an optical head device in which laser light of different wavelengths are used for DVD and CD, it was necessary to use different retarders for different wavelengths separately whereby the size of the device was increased. Further, in a case of using the two-wavelength semiconductor laser source, it was necessary to produce a desired phase difference with respect to two different kinds of wavelength by a single retarder, and therefore, a retarder capable of producing a desired phase difference depending on different wavelengths was expected.
It is an object of the present invention to provide a retarder capable of compensating a fluctuation of wavelength caused by a temperature change of light emitted from a semiconductor laser source, in particular, capable of producing stably a predetermined phase difference even in a high temperature region, and an optical element provided with such retarder.
Further, it is another object of the present invention to provide an optical element wherein a retarder to be installed in an optical head device has a plurality of functions, whereby the optical head device can be miniaturized.
Further, it is another object of the present invention to provide a retarder for producing a desired phase difference depending on different wavelengths and an optical element provided with such retarder in an optical head device wherein laser light having two or more different wavelengths are used as light sources.